Conductive antioxidant paint and graphite electrode

ABSTRACT

There are disclosed a conductive antioxidant paint suitably used as an antioxidant material for a graphite electrode used in arc furnaces such as electric steel-making furnaces, and a graphite electrode coated with the conductive antioxidant paint.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a conductive antioxidant paintand a graphite electrode, and more particularly, to a conductiveantioxidant paint especially suitable as an antioxidant material for agraphite electrode used in arc furnaces such as electric steel-makingfurnaces, and a graphite electrode coated with such a conductiveantioxidant paint.

[0002] Hitherto, artificial graphite electrodes have been used inconventional arc furnaces such as electric steel-making furnaces. Uponthe use, the graphite electrodes have been exposed to extremely severeconditions such as large electric current, high temperature, splashes ofmolten steel or the like. In particular, since ultrahigh-temperaturearcs are generated at a tip end of the electrode, the graphite electrodehas been exposed to a temperature as high as about 400 to about 3,000°C. As a result, the graphite electrode is readily oxidized and consumedby an oxidative gas entering through openings of the furnace.

[0003] In the electric steel-making furnaces, costs for the electrodeused therein is very high, so that the consumed electrode leads to largeeconomical loss. Upon the oxidation, 50 to 70% by weight of a consumedportion of the electrode is due to oxidative attack from a side surfaceof the electrode, and the arc itself has a less influence on thewear-out of the electrode. The wear-out of the electrode by oxidationbecomes more remarkable at the position closer to the tip end thereof,so that the electrode is tapered toward the tip end, resulting inaccelerated wear-out of the electrode in the longitudinal direction.Accordingly, if the oxidative attack from the side surface of theelectrode is sufficiently prevented, the wear-out of the electrode canbe effectively reduced with a large economical merits.

[0004] Conventionally, various methods have been proposed in order toinhibit the oxidation of the electrode. For example, there are known amethod of applying to the electrode an antioxidant paint composed of amatrix containing a glazing material (frit) having a melting point ofnot more than 1,000° C., and refractory aggregates (Japanese PatentApplication Laid-Open (KOKAI) No. 48-72211); a method of forming anon-conductive antioxidant paint layer on the surface of graphiteelectrode (Japanese Patent Application Laid-Open (KOKAI) No. 59-51499);a method of applying onto the electrode, a paint prepared by dispersingalumina or silica fine particles in a colloid solution containing silicaultrafine particles (Japanese Patent Application Laid-Open (KOKAI) No.3-45583); or the like.

[0005] However, since any of these conventional paints isnon-conductive, it is required that a chuck portion of the electrode isreleased from being coated therewith in order to ensure the current flowto the electrode. For this reason, there arise problems such ascomplicated coating processes, insufficient oxidation-resistant propertyat the uncoated portion of electrode or the like. In particular, in themethod described in Japanese Patent Application Laid-Open (KOKAT) No.48-72211, when the frit is softened at a temperature of not more than1,000° C., the coating film undergoes shrinkage, so that film defectssuch as peeling-off, penetration or ruptures tend to be caused. In orderto eliminate these film defects, there has been proposed a method ofrepeating the coating works until the thickness of the obtained coatingfilm after melting the frit becomes as large as about 1 mm. However, insuch a case, the working efficiency has been considerably deteriorated.

[0006] Further, in order to solve the above problems, there have beenproposed conductive antioxidant paints of the type which can be coatedeven on the chuck portion of electrode by imparting a conductivity tothe obtained coating film (Japanese Patent Application Laid-Open (KOKAI)Nos. 7-268248, 7-268249 and 7-268250). More specifically, JapanesePatent Application Laid-Open (KOKAI) No. 7-268248 describes a conductiveantioxidant material containing a refractory aggregate, a colloidalbinder and carbon black without any glass frits; Japanese PatentApplication Laid-Open (KOKAI) No. 7-268249 describes a conductiveantioxidant material containing a refractory aggregate, a binder andgraphitized carbon black; and Japanese Patent Application Laid-Open(KOKAI) No. 7-268250 describes a conductive antioxidant materialcontaining a refractory aggregate, a binder, carbon black and a polymeremulsion. The refractory aggregate includes oxides such as silica,alumina, titania or zirconia, and the binder includes inorganic colloid.

[0007] However, these conventional conductive antioxidant paintsgenerate a large amount of hydrogen gas during the storage ortransportation, resulting in poor storage stability thereof and breakageof a container therefor. For this reason, a long-period storage and along-distance transportation of these paints are difficult.

[0008] In addition, in the case where the conventional conductiveantioxidant paints are coated on the electrode, the obtained coatingfilm suffers from pinholes by the hydrogen gas generated therefrom, sothat the electrode tends to be burned from the pinholes. Thus, theconventional conductive antioxidant paints are deteriorated inantioxidant property when exposed to a temperature as high as not lessthan 1,000° C. As a result, the electrode is not only deteriorated inheat resistance and oxidation resistance, but also more likely to beconsumed by the oxidation.

[0009] Meanwhile, it is known that the temperature within the arcfurnace reaches not less than 1,000° C., thereby causing the followingproblems concerning the conventional conductive antioxidant paints. Thatis, the oxidation-resistant effect of the above paints is exhibited by aglass-like coating film formed by heat-melting film-forming componentsof the paints at a temperature of about 800° C. However, when the paintsare heated to a temperature as high as not less than 1,000° C., theobtained glass-like coating film is unsuitably lowered in viscosity andfallen off from the surface of the electrode, resulting in adiscontinuous glass-like coating film. Such discontinuous glass-likecoating film fails to prevent the contact between oxygen and theelectrode, thereby causing defects such as burning-in of the electrode,e.g., cissing thereon.

[0010] As a result of the present inventors' earnest studies to solvethe above problems, it has been found that (1) the generation ofhydrogen gas is caused when the paint has a high pH value, and is due toby the reaction between a trace amount of alkali components andimpurities of the metal compound; (2) the viscosity of the glass-likecoating film is lowered by melt-penetration of the alkali componentsinto the coating film; or (3) the generation of hydrogen gas is causedby the existence of specific elements in the paint. The presentinvention has been attained based on the above finding.

[0011] More specifically, it has been found that the deterioration inviscosity of the glass-like coating film is due to such a phenomenonthat alkali metals such as sodium and potassium or compounds thereofsuch as alkali metal oxides, and alkali earth metals such as calcium andmagnesium or compounds thereof such as alkali earth metal oxides aremelt-penetrated into the glass-like coating film after burning ofcarbon, and that the deteriorated viscosity of the glass-like coatingfilm causes falling-off or separation of the coating film from theelectrode, thereby adversely affecting the antioxidant effect of theelectrode. In addition, it has been found that the above alkalicomponents tend to react with the impurities of a metal compound togenerate a hydrogen gas, thereby considerably deteriorating a storagestability of the paint, and that when hydrogen gas is generated duringdrying step of the coating film, pinholes are formed therein, so thatthe electrode tends to be burned from the pinholes.

SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide an improvedconductive antioxidant paint which is inhibited from generating ahydrogen gas during the storage or transportation, thereby avoidingpossible breakage of a container therefor and imparting an excellentstorage stability, and which can prevent a graphite electrode used inarc furnace from being consumed by oxidation; and a graphite electrodewhich is coated with the conductive antioxidant paint.

[0013] Another object of the present invention is to provide aconductive antioxidant paint which exhibits an excellent antioxidanteffect even when exposed to a temperature as high as not less than1,0000° C., and a graphite electrode which is coated with such aconductive antioxidant paint.

[0014] A further object of the present invention is to provide aconductive antioxidant paint which has an excellent storage stability,is free from pinholes when formed into a coating film, and exhibits anexcellent antioxidant effect even when exposed to a temperature as highas not less than 1,000° C., and a graphite electrode which is coatedwith such a conductive antioxidant paint.

[0015] In a first aspect of the present invention, there is provided aconductive antioxidant paint comprising a conductive material, anantioxidant material, a polymer emulsion and an inorganic colloid as abinder, and a transition metal; and having a pH value of not more than9.

[0016] In a second aspect of the present invention, there is provided aconductive antioxidant-paint comprising a conductive material, anantioxidant material, a polymer emulsion and an inorganic colloid as abinder, and a transition metal, an alkali metal and/or alkali earthmetal content being not more than 20% by weight based on the weight ofthe antioxidant material.

[0017] In a third aspect of the present invention, there is provided aconductive antioxidant paint comprising a conductive material, anantioxidant material and a binder; and having a total content ofaluminum and silicon elements of not more than 1% by weight based on theweight of the solid content of the paint.

[0018] In a fourth aspect of the present invention, there is provided acoated graphite electrode comprising a graphite electrode coated withany of the conductive antioxidant paints described in the above aspects.

DETAILED DESCRIPTION OF THE INVENTION

[0019] First, the conductive antioxidant paint according to the presentinvention will be explained. The conductive antioxidant paint accordingto the present invention (hereinafter referred to merely as “paint”)includes a conductive material, an antioxidant material and a binder,and can form a coating film or layer exhibiting a good conductivity andantioxidant effect. Therefore, the paint of the present invention iseffective to prevent the oxidation of a graphite electrode when appliedthereon.

[0020] [Conductive Material]

[0021] The conductive material used in the present invention is notparticularly restricted as long as the material can impart a goodconductivity to the resultant coating film. Typical examples of theconductive materials may include carbon black and graphite. Theseconductive materials may be used alone or in the form of a mixturethereof.

[0022] As the carbon black, there may be used those produced by anysuitable methods such as a furnace method, an acetylene method, athermal method and a contact method. Among these carbon blacks,graphitized carbon black produced by treating carbon black in atemperature of not less than 2,000° C., preferably 2,500 to 3,000° C. insuch an atmosphere containing substantially no oxygen (e.g., under N₂gas flow, in vacuum or in carbon powder) is preferred, and thegraphitized carbon black having a divided value of crystallite thicknessLc (Å) to particle size (nm) of 1.0 to 3.0 is more preferred.

[0023] The carbon black used in the present invention preferablycontains an alkali metal such as potassium and sodium and/or an alkaliearth metal in an amount of not more than 1% by weight, more preferablynot more than 0.5% by weight. By limiting the alkali metal and/or alkaliearth metal content in carbon black to the above-specified range, itbecomes possible not only to increase the ignition point of carbon blackitself, but also to keep a good conductivity of the obtained coatingfilm until reaching near 400° C. in which the chuck portion of theelectrode is present.

[0024] As the graphite, there may be used any suitable commerciallyavailable products such as flake graphite, earthy graphite, artificialgraphite, expanded graphite or the like. Among them, preferred graphitesare those having an ash content of not more than 2% by weight,preferably not more than 1% by weight, more preferably not more than0.5% by weight.

[0025] The content of the conductive material is usually in the range of0.5 to 50% by weight, preferably 3 to 35% by weight based on the solidcontent of the paint.

[0026] When carbon black solely is used as the conductive material, thecontent of the carbon black is preferably 2 to 30% by weight, morepreferably 5 to 20% by weight based on the weight of the solid contentof the paint. When the content of the carbon black is less than 2% byweight, the obtained paint may be deteriorated in conductivity. When thecontent of the carbon black is more than 30% by weight, the viscosity ofthe obtained paint may tend to become too high, or the carbon blackcontained in a coating film may tend to be burnt, resulting indeteriorated antioxidant performance of the coating film.

[0027] Also, when graphite solely is used as the conductive material,the content of the graphite is preferably 0.5 to 20% by weight, morepreferably 3 to 15% by weight based on the weight of the solid contentof the paint. When the content of the graphite is less than 0.5% byweight, the resultant coating film may tend to be deteriorated in effectof improving the sliding property. When the content of the graphite ismore than 20% by weight, the viscosity of the obtained paint may tend tobecome too high.

[0028] When the combination of carbon black and graphite is used as theconductive material, the content thereof lies in the above-specifiedrange.

[0029] When the electrode is consumed during the operation of arcfurnace, a fresh electrode is connected to a top of the old electrode,and an electrode holder (clamping device) mounted on the old electrodeis moved in the longitudinal direction of the electrode up to apredetermined position of the fresh electrode. Upon the replacementstep, a coating film formed on the fresh electrode tends to be damagedby the contact with the electrode holder. In order to prevent thecoating film from being damaged (peeled) by the contact with theelectrode holder, an additional amount of graphite powder may becontained in the coating film, thereby improving a sliding resistancethereof and reducing the damage thereof. The amount of the graphitepowder blended is preferably 10 to 70 parts by weight, more preferably20 to 60 parts by weight based on 100 parts by weight of the antioxidantmaterial.

[0030] [Antioxidant Material]

[0031] The antioxidant material used in the present invention is notparticularly restricted as long as the material can impart a requiredantioxidant property to the obtained coating film. Examples of theantioxidant materials may include oxides such as silica, alumina,titania and zirconia; carbides such as SiC, B₄C, CrC, WC, TiC, VC, ZrCand NbC, and carbides of an element selected from the group consistingof Ge, Sb, Sn and Al; nitrides such as TiN, VN, NbN and ZrN, andnitrides of an element selected from the group consisting of B, Si, Ge,Sb, Sn and Al; silicides such as CrSi₂, TiSi₂ and ZrSi₂; borides such asZrB₂, TiB₂ and CrB; or a boron element as a single substance. The aboveoxides, carbides, nitrides, suicides and element may be used alone or inthe form of a mixture of any two or more thereof.

[0032] The antioxidant materials are vitrified when exposed to atemperature as high as not less than 1,000° C. and, therefore, can bereferred to as “glass-forming substance”. More specifically, theantioxidant materials are gradually converted into oxides under heatedcondition, and form a glass-like coating film while enclosing othercomponents such as inorganic colloid therein, so that the electrode isshielded and protected from oxygen by the thus formed film. Among theabove antioxidant materials, ZrB₂, B₄C, TiC, SiC and Si are preferredbecause these materials can impart an excellent oxidation-resistantproperty to the electrode and an excellent stability to the glass-likecoating film under heated condition. Further, when carbides are used asthe antioxidant material, more excellent oxidation-resistant effect canbe exhibited since the carbides are oxidized and serve for inhibitingoxygen from penetrating into the electrode. Among these antioxidantmaterials, the combination of B₄C and SiC is especially preferred.

[0033] The antioxidant materials contain impurities such as aluminum orsilicon in the form of a single substance (hereinafter referred tomerely as “aluminum element (metal) or silicon element”). It isconsidered that the hydrogen gas is generated by reacting theseimpurities with alkali components contained in the colloidal silica usedas a binder. The generation of hydrogen gas causes remarkabledeterioration in storage stability of the paint, and if the hydrogen gasis generated during drying step of the coating film, pinholes are formedin the coating film, resulting in burning of the electrode from thepinholes. Therefore, the antioxidant materials are preferablypre-treated with acids such as hydrochloric acid to reduce the contentsof aluminum and silicon elements therein. By reducing the contents ofaluminum and silicon elements in the antioxidant materials, it ispossible to inhibit the generation of hydrogen gas, enhance the keepingproperty of the coating solution, and prevent the formation of pinholesupon coating. The total content of aluminum and silicon elements ispreferably not more than 1% by weight, more preferably not more than0.5% by weight based on the weight of the solid content of the paint.

[0034] The acid treatment of the antioxidant materials is notparticularly restricted as long as the aluminum and silicon elements canbe effectively removed from antioxidant materials by preliminarilycontacting the antioxidant materials with acids. For example, there maybe exemplified a method wherein the acids and the antioxidant materialsare mixed and stirred together. In such a case, the mixing and stirringtime may be appropriately selected depending upon the amounts of thealuminum and silicon elements contained in the antioxidant materials,for example, usually not less than 10 hours, preferably 20 to 50 hours.

[0035] The acids used for the treatment is not particularly restrictedas long as the aluminum and silicon elements can be reacted therewith.Specific examples of the acids may include hydrochloric acid, sulfuricacid and phosphoric acid. The acid may be used in an excess amounthigher than the calculated amount based on amounts of the aluminum andsilicon elements contained in the antioxidant materials.

[0036] The amount of the antioxidant material contained in the paint isusually in the range of 10 to 90% by weight, preferably 15 to 45% byweight based on the weight of the solid content of the paint. When thecontent of the antioxidant material is less than 10% by weight, theobtained coating film may tend to be deteriorated in stability, therebycausing cissing and, therefore, promoting the oxidation of theelectrode. When the amount of the antioxidant material is more than 90%by weight, the obtained coating film may tend to be deteriorated inanchoring property. Especially, when exposed to a temperature of 400 to800° C., the electrode undergoes oxidation due to deteriorated functionof the coating film.

[0037] [Binder]

[0038] The binder used in the present invention is not particularlyrestricted as long as the material ensures a good continuity of theobtained coating film. Examples of the binders may include polymeremulsions and inorganic colloids. These binders may be used alone or inthe form of a mixture of any two or more thereof.

[0039] The polymer emulsion acts as a binder capable of enhancing acontinuity of the obtained coating film at a temperature between roomtemperature and 400° C., and improving a sliding property thereof. Assuch polymer emulsions, there may be exemplified rubber latexes, resinemulsions or the like. Specific examples of the polymer emulsions mayinclude emulsions of polymers produced by emulsion polymerization, oremulsions produced by re-emulsifying these polymers.

[0040] As the rubber latexes, there may be exemplified natural rubberlatexes and synthetic rubber latexes. Examples of the synthetic rubberlatexes may include butadiene polymers, styrene-butadiene copolymers,actylonitrile-butadiene copolymers, methyl methacrylate-butadienecopolymers, acrylate-based latexes or the like. Examples of the resinemulsions may include emulsions of polystyrene, styrene-acrylonitrilecopolymers, polyvinyl chloride, ethylene-vinyl acetate copolymers,polymethyl methacrylate, polyethylene or the like. These syntheticrubber latexes and resin emulsions may be respectively used alone or inthe form of a mixture of any two or more thereof. Among these polymeremulsions, styrene-butadiene copolymer latex is preferred, andcarboxyl-containing styrene-butadiene copolymer latex is more preferred.

[0041] The inorganic colloid also functions as a binder for stronglyanchoring the coating film onto the electrode at a temperature of about400 to about 600° C. Examples of such inorganic colloids may includecolloidal silica, colloidal alumina, colloidal zirconia or the like.Also, inorganic colloid precursors such as tetraethyl orthosilicatewhich are capable of forming inorganic colloid by adding an acid such asHCl thereto, may be used together with the acid. Among these inorganiccolloids, colloidal silica is preferred. The silica particles used inthe inorganic colloid preferably have an average particle size of notmore than 100 nm. When the average particle size of the silica particlesis more than 100 nm, the inorganic colloid may be deteriorated infunctionality as a binder.

[0042] In general, the colloidal silica is synthesized by desaltingwater glass and, therefore, contains residual alkali components such assodium and potassium in an amount of 1 to 10% by weight in the aqueoussolution. For this reason, it is considered that when such a colloidalsilica is used, the alkali components derived therefrom are reacted withthe aluminum or silicon elements present in the antioxidant material togenerate a hydrogen gas. As a result, it is suggested that the paint isconsiderably deteriorated in storage stability, and the coating filmobtained therefrom suffers from pinholes due to the hydrogen gasgenerated upon drying of the coating film, resulting in burning of theelectrode from the pinholes. Thus, in order to use such a colloidalsilica in the present invention, it is preferred that the colloidalsilica is further subjected to desalting treatment so as to control theconcentration of the alkali components such as sodium and potassium tonot more than 1% by weight, or water contained in the colloidal silicais replaced with an organic solvent such as methanol and ethyleneglycol. The use of the thus pre-treated colloidal silica can inhibit thegeneration of hydrogen gas, enhance the storage stability of the paint,and prevent the formation of pinholes in the coating film.

[0043] The amount of the alkali metal and/or alkali earth metalcontained in the inorganic colloid is preferably not more than 1% byweight, more preferably not more than 0.5% by weight.

[0044] The amount of the binder used is usually 0.05 to 40% by weight,preferably 1 to 25% by weight (calculated as solid content, i.e.,residual non-volatile components produced when one gram of the binder isheated in air at 150° C. for one hour) based on the solid content of thepaint.

[0045] The content of the polymer emulsion is preferably 0.05 to 10% byweight (calculated as solid content of the emulsion, i.e., residualnon-volatile components produced when one gram of the emulsion is heatedin air at 150° C. for one hour) based on the weight of the paint. Whenthe content of the polymer emulsion is less than 0.05% by weight, theobtained paint may tend to be deteriorated in sliding resistance. Whenthe content of the polymer emulsion is more than 10% by weight, thepaint may tend to be deteriorated in stability with the passage of time.

[0046] The content of the inorganic colloid is preferably 1 to 30% byweight, more preferably 2 to 15% by weight (calculated as solid contentof the inorganic colloid, i.e., residual non-volatile componentsproduced when one gram of the inorganic colloid is heated in air at 150°C. for one hour) based on the weight of the paint. The content of theinorganic colloid is preferably as small as possible from the standpointof a good conductivity. However, when the content of the inorganiccolloid is less than 2% by weight, the paint exhibits substantially noadhesion force, so that the coating film obtained therefrom tends to beseparated from the electrode. On the other hand, when the content of theinorganic colloid is more than 30% by weight, the paint exhibits a goodadhesion force, but is deteriorated in conductivity, resulting ingeneration of sparks when applied on the electrode.

[0047] In the case where the combination of the polymer emulsion and theinorganic colloid is used as the binder, the content thereof lies in theabove-specified range.

[0048] [Other Components]

[0049] The paint of the present invention may further contain atransition metal in addition to the above-described components. Thetransition metal has a function for enhancing a wettability of theglass-like coating film formed of the antioxidant material, relative tothe electrode. More specifically, the transition metal shows an effectof increasing a wettability of the glass-like coating film relative tothe surface of the graphite electrode upon vitrification of silica undera temperature of not less than 1,000° C. That is, the transition metalmaintains a continuity of the glass-like coating film by uniformlywetting the surface of the graphite electrode and preventing the coatingfilm from being separated into ball-like pieces, i.e., spheroidized.

[0050] As the transition metals, there may be-used elements (metals)such as chromium, tungsten, titanium, cobalt or the like. From thestandpoints of safety and inexpensiveness, the use of chromium, titaniumand tungsten elements (metals) is preferred.

[0051] The amount of the transition metal blended in the paint is notless than 0.1% by weight based on the solid content of the paint, inorder to exhibit an excellent effect thereof. The amount of thetransition metal blended is preferably in the range of 0.1 to 70% byweight, more preferably 2 to 40% by weight, based on the weight of thesolid content of the paint. When the amount of the transition metalblended is less than 0.1% by weight, the obtained vitrified coating filmmay tend to be deteriorated in adhesion to the graphite electrode,resulting in occurrence of cissing and, therefore, pooroxidation-resistant property. When the amount of the transition metalblended is more than 70% by weight, it may be difficult to maintain acontinuity of the coating film obtained by the vitrification ofaggregates or the like.

[0052] Also, the conductive antioxidant paint of the present inventionmay further contain antiseptic agents as well as various other additivessuch as defoaming agents, leveling agents, antiprecipitants or the likein order to enhance a keeping property thereof.

[0053] Next, the process for producing the conductive antioxidant paintof the present invention will be described below.

[0054] The conductive antioxidant paint of the present invention may beproduced by blending the above-described components together by thefollowing method. That is, the conductive antioxidant paint of thepresent invention is produced by mixing the respective components witheach other in water or an organic solvent, and, if required, subjectingthe obtained mixture to dispersing treatment. The-amount of water or theorganic solvent blended may be appropriately adjusted before coatingdepending on coating methods used. For example, the amount of water orthe organic solvent blended is preferably 20 to 200 parts by weight,more preferably 30 to 80 parts by weight based on 100 parts by weight ofthe solid content of the conductive antioxidant paint.

[0055] The organic solvents usable in the present invention are notparticularly restricted as long as the respective components of thepaint can be dispersed therein. Specific examples of the organicsolvents may include alcohol-based solvents such as methanol,isopropanol, isobutanol, isopentanol, ethylene glycol, ethylene glycolmonopropyl ether, ethylene glycol monomethyl ether, ethylene glycolmonobutyl ether and ethylene glycol monoethyl ether; hydrocarbon-basedsolvents such as n-hexane, heptane, xylene, toluene, cyclohexane,naphtha and styrene; ketone-based solvents such as acetone, methylisobutyl ketone, methyl ethyl ketone, isophorone and acetophenone;amide-based solvents such as dimethylacetamide and methyl pyrrolidone;and ester-based solvents such as methyl acetate, ethyl acetate, isobutylacetate, octyl acetate, acetic acid ethylene glycol monomethyl ether andacetic acid diethylene glycol monomethyl ether.

[0056] The dispersing medium used may be appropriately selecteddepending on the aims and environments upon use. The conductiveantioxidant paint of the present invention is applied onto the electrodeused under a high temperature condition. Therefore, when the coatingoperation is conducted near the position where the electrode ispractically used, the use of less flammable organic solvents ispreferred from the standpoint of inhibiting fire accidents. In thiscase, the content of solvents other than water is preferably controlledto as low a level as possible. For example, the content of solventsother than water. i.e., volatile solvents, is preferably not more than10% by weight, more preferably not more than 5% by weight.

[0057] Alternatively, volatile solvents may be used to promote drying ofthe coating film. In such a case, the volatile solvents other than watermay be contained in an amount of not less than 5% by weight, ifpossible, not less than 10% by weight.

[0058] The dispersing method is not particularly restricted as long asthe respective components can be dispersed to a sufficient extent. Forexample, the dispersion treatment of a mixture composed of the aboverespective components may be conducted using dissolver, homomixer, ballmill, roll mill, attritor, Dyno-mill, sand mills such as pico mill,basket mill, easy mill and LMZ-SC mill, ultrasonic dispersing apparatus,homogenizers such as altimizer, nanomizer and micro fluidizer, or jetmill.

[0059] Next, properties of the conductive antioxidant paint will bedescribed below. The conductive antioxidant paint of the presentinvention shows any of the following properties.

[0060] [pH Value of Paint]

[0061] The conductive antioxidant paint of the present invention has apH value of not more than 9.0. Here, the pH value of the paint means apH value measured at ordinary temperature under such a condition inwhich the respective components are sufficiently stirred so as not toprecipitate. That is, in the present invention, the amount of thewater-soluble alkali components contained in the paint is defined by thepH value. When the pH value of the paint is not more than 9.0, theamount of the water-soluble alkali components contained in the paint canbe limited to an appropriate range, thereby inhibiting the generation ofhydrogen gas due to the reaction with impurities contained in the metalcompound. As a result, the conductive antioxidant paint of the presentinvention shows an excellent storage stability, can prevent theformation of pinholes when formed into a coating film, and can exhibitan excellent oxidation-resistant property even when exposed to as high atemperature as not less than 1,000° C. In order to further inhibit thegeneration of hydrogen gas, it is preferred that the pH value of thepaint is not more than 8.5 (lower limit thereof is usually 3).

[0062] As a method of controlling the pH value of the paint to not morethan 9.0, there may be used a method of appropriately selectingmaterials used and varying the mixing ratios therebetween, a method ofconverting the alkali components into salts thereof by adding acidsubstances thereto, a method of removing the alkali components byion-exchange treatment, or the like.

[0063] In particular, in the case of components containing alkali metalsand/or alkali earth metals, such as carbon black, inorganic colloid,etc., there may be selectively used those having a less content of thesealkali components.

[0064] [Content of Alkali Metals and/or Alkali Earth Metals]

[0065] As to the conductive antioxidant paint of the present invention,the content of alkali metals and/or alkali earth metals is not more than20% by weight based on the weight of the antioxidant material. Bylimiting the content of these alkali components based on the weight ofthe antioxidant material to the above range, the glass-like coating filmproduced from the antioxidant material at a high temperature isprevented from being deteriorated in viscosity, thereby maintaining acontinuity of the glass-like coating film. As a result, the coating filmproduced from the paint of the present invention can exhibit anexcellent oxidation-resistant property even at a temperature as high asnot less than 1,000° C.

[0066] The content of the alkali metals and/or alkali earth metals asdefined herein is concerned with not individual alkali contents of therespective components, but a total amount of alkali components containedin the paint based on the weight of the antioxidant material. This isbecause the glass-like coating film is substantially composed of theantioxidant material solely which is a residual component only presentafter exposed to a temperature of not less than 1,000° C., and as aresult, the viscosity of the glass-like coating film is varied, i.e.,influenced by the amount of alkali components contained in theglass-like coating film. In order to allow the obtained coating film toexhibit an excellent oxidation-resistant property, the content of thealkali metals and/or alkali earth metals contained in the antioxidantmaterial is preferably not more than 15% by weight, more preferably notmore than 10% by weight based on the weight of the antioxidant material.

[0067] Accordingly, in the case of components containing alkali metalsand/or alkali earth metals, such as carbon black, inorganic colloid,etc., there may be selectively used those having a less content of thesealkali components. The amount of the alkali metals and/or alkali earthmetals contained in these components is usually not more than 1% byweight, preferably not more than 0.5% by weight.

[0068] In order to control the amount of the alkali metals and/or alkaliearth metals contained in the antioxidant material to not more than 20%by weight based on the weight of the antioxidant material, the kind andmixing ratio of the antioxidant material may be appropriately selected.

[0069] [Content of Aluminum and Silicon]

[0070] As to the conductive antioxidant paint of the present invention,the content of aluminum and silicon elements contained in the paint isnot more than 1% by weight based on the weight of the solid content ofthe paint.

[0071] The amounts of aluminum and silicon elements contained in thepaint can be readily calculated from aluminum and silicon contents ofthe respective components blended, if known. Even though the aluminumand silicon contents of the respective components are not known, theamounts of aluminum and silicon elements contained in the paint can beobtained by the following method.

[0072] That is, 5 g of the paint is heated in air at 150° C. for 3 hoursto measure an amount of combustion residues as the solid contentthereof. In addition, a total amount of aluminum and silicon elementscontained in the solid content of the paint is measured by X-raydiffraction method to obtain a weight percentage thereof based on thesolid content of the paint.

[0073] By controlling the total amount of aluminum and silicon elementsto not more than 1% by weight based on the solid content of the paint,it is possible to limit the amount of hydrogen gas generated from thepaint to substantially ignorable level, safely store and transport thepaint, and prevent the formation of pinholes when formed into a coatingfilm. The total amount of aluminum and silicon elements contained in thesolid content of the paint is preferably not more than 0.5% by weight,more preferably not more than 0.1% by weight based on the solid contentof the paint, thereby further inhibiting the generation of hydrogen gas.

[0074] In order to control the total amount of aluminum and siliconelements to not more than 1% by weight based on the solid content of thepaint, there may be used a method of preparing a paint by mixing theabove respective components together, and then removing aluminum andsilicon elements from the obtained paint. Preferably, the total amountof aluminum and silicon elements can be reduced by the method ofpreviously removing these elements from the respective components beforeblending.

[0075] [Formation of Coating Film]

[0076] The conductive antioxidant paint of the present invention isapplied onto the side surface and chuck portion of an electrode used inarc furnace such that the thickness of the obtained coating film isusually about 100 to about 500 μm (after drying).

[0077] The conductive antioxidant paint of the present invention may beapplied by an optimum method selected from general coating methods suchas an immersion-coating method, a brush-coating method, a spray-coatingmethod, an electrostatic coating method or the like. In this case, theviscosity of the conductive antioxidant paint may be suitably controlledaccording to the coating method used. Also, the coating operation may berepeated a plurality of times to form an overlapped coating film.Further, different kinds of paints may be applied two or more times.

[0078] The conductive antioxidant paint of the present invention showsan excellent storage stability, prevents the formation of pinholes whenformed into a coating film, and exhibits an excellentoxidation-resistant effect even when exposed to as high a temperature asnot less than 1,000° C., especially not less than 1,200° C.

[0079] Also, the conductive antioxidant paint of the present inventioncan be applied onto s chuck portion of the electrode, and can exhibit anexcellent sliding resistance as well as an excellent oxidation-resistanteffect even when exposed to as high a temperature as not less than1,000° C.

[0080] Further, the graphite electrode of the present invention isprevented from being consumed by oxidation.

EXAMPLES

[0081] The present invention will be described in more detail byreference to the following examples. However, these examples are onlyillustrative and not intended to limit the present invention thereto.

Examples 1 to 5 and Comparative Examples 1 to 4:

[0082] First, ten pairs of paints (a) and (b) were prepared bydispersing the respective components shown in (a) and (b) of Tables 1and 2, for 60 minutes using a sand grinder. Carbon black, colloidalsilica and dispersant used in each composition are shown in Table 3.

[0083] In Comparative Example 1, an alkali component (MgO) waspositively added to the composition; in Comparative Example 2, nopolymer emulsion (latex emulsion) was added; in Comparative Example 3,no inorganic colloid (colloidal silica) was added; and in ComparativeExample 4, no transition metal (chromium powder) was added.

[0084] Then, the paint (a) was applied in an amount of 100 to 250 g/m²(after drying) onto a whole surface of a test piece sliced from agraphite electrode by a brush-coating method, and dried for one hour.Successively, the paint (b) was applied in an amount of 250 to 500 g/m²onto the test piece. The thus obtained coated test piece was allowed tostand over night for drying, and then the surface resistivity thereofwas measured by a tester. Thereafter, the test piece was allowed tostand in an ashing furnace (manufactured by Hayashi Denko Co., Ltd.) at400° C. for 30 minutes, and the change in surface resistivity wasmeasured. Then, the test piece was baked at 1,000° C. for 30 minutes,cooled in air to room temperature, and then observed to examine thecondition of the coating film. The results are shown in Table 4.

[0085] In Examples 1 to 5, although some of the paints after baked at1,000° C. were colored slightly green by the oxidation of chromium, itwas confirmed that the paints were free from occurrence of cissing, werecapable-of forming a uniform coating film, and exhibited a sufficientoxidation-resistant effect.

[0086] In Comparative Example 1, the paint exhibited a very goodcondition upon measurement of the surface resistivity at 400° C.However, when baked at 1,000° C., a non-uniform coating film was formed,and there were observed many burnt-in portions.

[0087] In Comparative Example 2, upon measurement of the surfaceresistivity at 400° C., the obtained coating film was brittle, andreadily fallen off from the electrode. When baked at 1,000° C., anon-uniform coating film was formed, and there were observed manyburnt-in portions.

[0088] In Comparative Example 3, upon measurement of the surfaceresistivity at 400° C., the obtained coating film was brittle, andreadily fallen off from the electrode. Further, many fractures wereobserved on the coating film. When baked at 1,000° C., it was confirmedthat the obtained coating film was discontinuous, so that clearanceswere formed between the coating film and the electrode, and theelectrode was severely damaged. Therefore, the paint failed to show anoxidation-resistant effect when exposed to a temperature as high as notless than 1,000° C.

[0089] In Comparative Example 4, upon measurement of the surfaceresistivity at 400° C., the obtained coating film was kept in a verygood condition. However, when baked at 1,000° C., the coating film wasdiscontinuous and suffered from cissing, and the electrode was damaged.

[0090] As apparent from the above, the graphite electrodes coated withthe paints of the present invention still maintained a good conductivityat 400° C., and exhibited a good oxidation-resistant effect even whenexposed to 1,000° C.

Examples 6 to 10 and Comparative Examples 5 to 8:

[0091] First, ten pairs of paints (a) and (b) were prepared bydispersing the respective components shown in (a) and (b) of Tables 5and 6, for 60 minutes using a sand grinder. Carbon black, colloidalsilica and dispersant used in each composition are shown in Table 7.

[0092] In Comparative Example 5, colloidal silica 5 having a high sodiumcontent was used; in Comparative Example 6, no polymer emulsion (latexemulsion) was added; in Comparative Example 7, no inorganic colloid(colloidal silica) was added; and in Comparative Example 8, notransition metal (chromium powder) was added.

[0093] Then, the paint (a) was applied in an amount of 100 to 250 g/m²(after drying) onto a whole surface of a test piece sliced from agraphite electrode by a brush-coating method, and then dried for onehour. Successively, the paint (b) was applied in an amount of 250 to 500g/m² onto the test piece. The thus obtained coated test piece wasallowed to stand over night for drying, and then the surface resistivitythereof was measured by a tester. Thereafter, the test piece was allowedto stand in an ashing furnace (manufactured by Hayashi Denko Co., Ltd.)at 400° C. for 30 minutes, and the change in surface resistivity wasmeasured. Then, the test piece was baked at 1,000° C. for 30 minutes,cooled in air to room temperature, and then observed to examine thecondition of the coating film. The results are shown in Table 8.

[0094] In Examples 6 to 10, although some of the paints after beingbaked at 1,000° C. were colored slightly green by the oxidation ofchromium, it was confirmed that each paint was able to produce a uniformglass-like coating film without cissing, and exhibited a sufficientoxidation-resistant effect.

[0095] In Comparative Example 5, the paint generated a large amount ofhydrogen gas during its storage. After being baked at 1,000° C. for 30minutes, it was confirmed that the coating film suffered from pinholesdue to the removal of hydrogen gas, and the electrode was burnt-in fromthe pinholes.

[0096] In Comparative Example 6, upon measurement of the surfaceresistivity at 400° C., the obtained coating film was brittle, andfallen off from the electrode. When being based at 1,000° C., it wasconfirmed that the obtained coating film was non-uniform, and there wereobserved many burnt-in portions on the electrode.

[0097] In Comparative Example 7, upon measurement of the surfaceresistivity at 400° C., the obtained coating film was brittle, andreadily fallen off from the electrode. Further, many fractures wereobserved on the coating film. When being baked at 1,000° C., it wasconfirmed that the obtained coating film was discontinuous, so thatclearances were formed between the coating film and the electrode, andthe electrode was severely damaged. Therefore, the paint failed to showan oxidation-resistant effect when exposed to a temperature as high asnot less than 1,000° C.

[0098] In Comparative Example 8, upon measurement of the surfaceresistivity at 400° C., the obtained coating film was kept in a verygood condition. However, when being baked at 1,000° C., the coating filmwas discontinuous and suffered from cissing, and the electrode wasdamaged.

Example 11 and Comparative Example 9:

[0099] First, two pairs of paints (a) and (b) were prepared bydispersing the respective components shown in (a) and (b) of Table 9,for 60 minutes using a sand grinder. Carbon black, colloidal silica,dispersant and SiC used in each composition were “#4000B” produced byMitsubishi Kagaku Co., Ltd., “FINE CATALLOID SBB-120” (solid content:20% by weight) produced by Shokubai Kasei Kogyo Co., Ltd., “DEMOLE N”produced by Kao Co., Ltd., and “DIASIC CF-12OF” produced by YakushimaDenko Co., Ltd., respectively. The SiC had a purity of 95%, andcontained as impurities, metal aluminum and silicon element in a totalamount of 5% by weight.

[0100] The solid contents of the paints (a) and (b) were 67.1% by weightand 62.2% by weight, respectively. The contents of the aluminum andsilicon elements were 1.48% by weight for the paint (a) and 1.33% byweight for the paint (b) based on the weight of the solid content ofeach paint.

[0101] In Example 11, treated SiC was used; and in Comparative Example9, untreated SiC was used. The SiC was treated as follows. That is,equivalent amounts of SiC and 0.1N HCl were charged into a beaker, andthe mixture was stirred for 24 hours. Then, the resultant mixture wasstirred by “HOT PLATE & STIRRER” (tradename) manufactured by CorningCorp., to evaporate water therefrom.

[0102] The paint containing the thus treated SiC was measured by X-raydiffraction method to determine the contents of aluminum and siliconcontained in the solid content of the paint. However, the contents ofaluminum and silicon were less than the detection lower limit and,therefore, undetectable.

[0103] The generation of hydrogen gas from the obtained paint wascontinuously observed from the preparation stage thereof. The resultsare shown in Table 10.

[0104] Then, the paint (a) was applied in an amount of 100 to 250 g/m²(after drying) onto a whole surface of a test piece sliced from agraphite electrode by a brush-coating method, and then dried for onehour. Successively, the paint (b) was applied in an amount of 250 to 500g/m² onto the test piece. The thus obtained coated test piece wasallowed to stand over night for drying, and then the surface resistivitythereof was measured by a tester. Thereafter, the test piece was allowedto stand in an ashing furnace (manufactured by Hayashi Denko Co., Ltd.)at 400° C. for 30 minutes, and the change in surface resistivity wasmeasured. Then, the test piece was baked at 1,000° C. for 30 minutes,cooled in air to room temperature, and then observed to examine thecondition of the coating film. The results are shown in Table 11.

[0105] In both of Example 11 and Comparative Example 9, although some ofthe paints after being baked at 1,000° C. were colored slightly green bythe oxidation of chromium, it was confirmed that each paint was able toproduce a uniform glass-like coating film without cissing., andexhibited a sufficient oxidation-resistant effect.

[0106] However, in Comparative Example 9, the paint suffered from thegeneration of hydrogen gas after the elapse of about one month. On thecontrary, the paint obtained in Example 11 was free from the generationof hydrogen gas even after the elapse of one month. TABLE 1 Example 1Example 2 Example 3 a b a b a b Carbon black 9.1 10.1 9.0 10.1 9.5 10.1Graphite powder 3.3 1.3 3.3 15.0 3.3 10.0 SiC powder 19.8 16.6 20.0 15.022.0 12.0 B4C powder 2.0 12.5 3.0 15.0 1.5 20.0 Latex emulsion 1.1 1.21.1 1.0 1.1 1.0 Colloidal silica 1 18.0 19.9 — — — — Colloidal silica 2— — 26.2 29.3 — — Colloidal silica 3 — — — — 26.2 29.4 Colloidal silica4 — — — — — — Tetraethyl orthosilicate — — — — — — HCl (0.1N) — — — — —— Silica gel — — — — — — Zirconia sol — — — — — — Cr powder 29.0 2.420.0 2.5 10 1.0 MgO — — — — — — Dispersant 1.0 1.1 1.0 1.1 1.0 1.1Ion-exchanged water 36.7 43.1 28.3 33.7 26.3 33.6 Alkali/antioxidant(wt. %) 0.56 0.47 0.66 0.56 0.65 0.53 Example 4 Example 5 a b a b Carbonblack 9.1 10.1 9.1 10.1 Graphite powder 3.3 1.2 3.3 13.0 SiC powder 19.815.0 15.0 18.0 B4C powder 1.9 13.0 1.8 14.0 Latex emulsion 1.1 1.0 1.11.0 Colloidal silica 1 — — — — Colloidal silica 2 — — — — Colloidalsilica 3 — — — — Colloidal silica 4 29.2 32.6 — — Tetraethylorthosilicate — — 5.5 6.0 HCl (0.1N) — — 10.7 10.7 Silica gel — — — —Zirconia sol — — — — Cr powder 29.0 2.4 0.1 0.05 MgO — — — — Dispersant1.0 1.1 1.0 1.1 Ion-exchanged water 27.2 30.5 81.0 87.0Alkali/antioxidant (wt. %) 0.73 0.59 0.59 0.34

[0107] TABLE 2 Comparative Comparative Example 1 Example 2 a b a bCarbon black 9.1 10.1 9.1 10.1 Graphite powder 3.3 1.3 3.3 1.3 SiCpowder 19.8 16.6 19.8 16.6 B4C powder 2.0 12.5 2.0 12.5 Latex emulsion1.1 1.2 — — Colloidal silica 1 18.0 19.9 18.0 19.9 Colloidal silica 2 —— — — Colloidal silica 3 — — — — Colloidal silica 4 — — — — Tetraethylorthosilicate — — — — HCl (0.1N) — — — — Silica gel — — — — Zirconia sol— — — — Cr powder 29.0 2.4 29.0 2.4 MgO 9.1 10.1 — — Dispersant 1.0 1.11.0 1.1 Ion-exchanged water 36.7 43.1 36.7 43.1 Alkali/antioxidant (wt.%) 25.6 20.8 0.56 0.47 Comparative Comparative Example 3 Example 4 a b ab Carbon black 9.1 10.1 9.1 10.1 Graphite powder 3.3 1.3 3.3 1.3 SiCpowder 19.8 16.6 19.8 16.6 B4C powder 2.0 12.5 2.0 12.5 Latex emulsion1.1 1.2 1.1 1.2 Colloidal silica 1 — — 18.0 19.9 Colloidal silica 2 — —— — Colloidal silica 3 — — — — Colloidal silica 4 — — — — Tetraethylorthosilicate — — — — HCl (0.1N) — — — — Silica gel 5.5 6.0 — — Zirconiasol — — — — Cr powder 29.0 2.4 — — MgO — — — — Dispersant 1.0 1.1 1.01.1 Ion-exchanged water 36.7 43.1 36.7 43.1 Alkali/antioxidant (wt. %)0.38 0.38 0.56 0.47

[0108] TABLE 3 Content of alkali metal or Average alkali particle earthsize metal Product and Solvent (nm) (wt. %) maker Carbon black — — ≧0.1“#4000B” produced by Mitsubishi Chemical Corp. Colloidal Methanol 10 to20 0.13 “METHANOL silica 1 SILICA SOL” produced by Nissan Kagaku Co.,Ltd. Colloidal water 10 to 20 0.2 “SNOWTEX O” silica 2 produced byNissan Kagaku Co., Ltd. Colloidal water about 50 0.2 “SNOWTEX OL” silica3 produced by Nissan Kagaku Co., Ltd. Colloidal water about 100 0.15 to“SPHERICA- silica 4 0.2 SLURRY 120” produced by Shokubai Kasei KogyoCo., Ltd. Dispersant (Na — — 10 “DEMOLE” salt of produced by Kaoformalin Co., Ltd. condensate of β-naphthalene sulfonic acid

[0109] TABLE 4 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Surface resistivity (Ω) 3 44 2 3 after coating Surface resistivity (Ω) 2 2 2 2 3 after treated at400° C. for 30 min. Condition after baked at ◯ ◯ ◯ ◯ ◯ 1,000° C. for 30min. Com. Com. Com. Com. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Surface resistivity (Ω)13 3 8 2 after coating Surface resistivity (Ω) 12 2 5 2 after treated at400° C. for 30 min. Condition after baked at x x x x 1,000° C. for 30min.

[0110] TABLE 5 Example 6 Example 7 Example 8 a b a b a b Carbon black9.1 10.1 9.0 10.1 9.5 10.1 Graphite powder 3.3 1.3 3.3 15.0 3.3 10.0 SiCpowder 19.8 16.6 20.0 15.0 22.0 12.0 B4C powder 2.0 12.5 3.0 15.0 1.520.0 Latex emulsion 1.1 1.2 1.1 1.0 1.1 1.0 Colloidal silica 1 18.0 19.9— — — — Colloidal silica 2 — — 26.2 29.3 — — Colloidal silica 3 — — — —26.2 29.4 Colloidal silica 4 — — — — — — Colloidal silica 5 — — — — — —Tetraethyl orthosilicate — — — — — — HCl (0.1N) — — — — — — Silica gel —— — — — — Zirconia sol — — — — — — Cr powder 29.0 2.4 20.0 2.5 10 1.0Dispersant 1.0 1.1 1.0 1.1 1.0 1.1 Ion-exchanged water 36.7 43.1 28.333.7 26.3 33.6 Alkali/antioxidant (wt. %) 0.56 0.47 0.66 0.56 0.65 0.53pH value 7.2 7.0 7.3 7.2 7.5 7.3 Example 9 Example 10 a b a b Carbonblack 9.1 10.1 9.1 10.1 Graphite powder 3.3 1.2 3.3 13.0 SiC powder 19.815.0 15.0 18.0 B4C powder 1.9 13.0 1.8 14.0 Latex emulsion 1.1 1.0 1.11.0 Colloidal silica 1 — — — — Colloidal silica 2 — — — — Colloidalsilica 3 — — — — Colloidal silica 4 29.2 32.6 — — Colloidal silica 5 — —— — Tetraethyl orthosilicate — — 5.5 6.0 HCl (0.1N) — — — — Silica gel —— — — Zirconia sol — — — — Cr powder 29.0 2.4 0.1 0.05 Dispersant 1.01.1 1.0 1.1 Ion-exchanged water 27.2 30.5 81.0 87.0 Alkali/antioxidant(wt. %) 0.73 0.59 0.59 0.34 pH value 7.5 6.4 6.0 4.8

[0111] TABLE 6 Comparative Comparative Example 5 Example 6 a b a bCarbon black 9.1 10.1 9.1 10.1 Graphite powder 3.3 1.3 3.3 1.3 SiCpowder 19.8 16.6 19.8 16.6 B4C powder 2.0 12.5 2.0 12.5 Latex emulsion1.1 1.2 — — Colloidal silica 1 18.0 19.9 18.0 19.9 Colloidal silica 2 —— — — Colloidal silica 3 — — — — Colloidal silica 4 — — — — Colloidalsilica 5 25.9 29.0 — — Tetraethyl orthosilicate — — — — HCl (0.1N) — — —— Silica gel — — — — Zirconia sol — — — — Cr powder 29.0 2.4 29.0 2.4Dispersant 1.0 1.1 1.0 1.1 Ion-exchanged water 36.7 43.1 36.7 43.1Alkali/antioxidant (wt. %) 1.65 1.37 0.56 0.47 pH value 9.8 9.7 7.0 6.8Comparative Comparative Example 7 Example 8 a b a b Carbon black 9.110.1 9.1 10.1 Graphite powder 3.3 1.3 3.3 1.3 SiC powder 19.8 16.6 19.816.6 B4C powder 2.0 12.5 2.0 12.5 Latex emulsion 1.1 1.2 1.1 1.2Colloidal silica 1 — — 18.0 19.9 Colloidal silica 2 — — — — Colloidalsilica 3 — — — — Colloidal silica 4 — — — — Colloidal silica 5 — — — —Tetraethyl orthosilicate — — — — HCl (0.1N) — — — — Silica gel 5.5 6.0 —— Zirconia sol — — — — Cr powder 29.0 2.4 — — Dispersant 1.0 1.1 1.0 1.1Ion-exchanged water 36.7 43.1 36.7 43.1 Alkali/antioxidant (wt. %) 0.380.38 0.56 0.47 pH value 7.2 7.1 7.2 7.1

[0112] TABLE 7 Content of alkali metal or Average alkali particle earthsize metal Product and Solvent (nm) (wt. %) maker Carbon black — — ≧0.1“#4000B” produced by Mitsubishi Chemical Corp. Colloidal Methanol 10 to20 0.13 “METHANOL silica 1 SILICA SOL” produced by Nissan Kagaku Co.,Ltd. Colloidal water 10 to 20 0.2 “SNOWTEX O” silica 2 produced byNissan Kagaku Co., Ltd. Colloidal water about 50 0.2 “SNOWTEX OL” silica3 produced by Nissan Kagaku Co., Ltd. Colloidal water about 100 0.15 to“SPHERICA- silica 4 0.2 SLURRY 120” produced by Shokubai Kasei KogyoCo., Ltd. Colloidal water ≧10 1 “CATALLOID 550” silica 5 produced byShokubai Kasei Kogyo Co., Ltd. Dispersant (Na — — 10 “DEMOLE” salt ofproduced by Kao formalin Co., Ltd. condensate of β-naphthalene sulfonicacid

[0113] TABLE 8 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Surface resistivity (Ω) 34 4 2 3 after coating Surface resistivity (Ω) 2 2 2 2 3 after treated at400° C. for 30 min. Condition after baked at ◯ ◯ ◯ ◯ ◯ 1,000° C. for 30min. Com. Com. Com. Com. Ex. 5 Ex. 6 Ex. 7 Ex. 8 Surface resistivity (Ω)4  3 8 2 after coating Surface resistivity (Ω) 2.5 2 5 2 after treatedat 400° C. for 30 min. Condition after baked at x x x x 1,000° C. for 30min.

[0114] TABLE 9 Comparative Example 11 Example 9 a b a b Carbon black 9.110.1 9.1 10.1 Graphite powder 3.3 13.0 3.3 13.0 SiC powder (treated)19.8 16.6 — — SiC powder (untreated) — — 19.8 16.6 B4C powder 1.9 12.51.9 12.5 Latex emulsion 1.1 1.2 1.1 1.2 Colloidal silica 23.2 26.0 23.226.0 Cr powder 29.3 2.5 29.3 2.5 Dispersant 1.0 1.1 1.0 1.1Ion-exchanged water 11.3 17.0 11.3 17.0

[0115] TABLE 10 After one day After one week After one month Example 11◯ ◯ ◯ Comparative ◯ ◯ x Example 9

[0116] TABLE 11 Comparative Example 11 Example 9 Surface resistivity (Ω)2 3 after coating Surface resistivity (Ω) 2 3 after treated at 400° C.for 30 min. Condition after baked at ◯ ◯ 1,000° C. for 30 min.

What is claimed is:
 1. A conductive antioxidant paint comprising a conductive material, an antioxidant material, a polymer emulsion and an inorganic colloid as a binder, and a transition metal; and having a pH value of not more than
 9. 2. A conductive antioxidant paint according to claim 1, wherein said antioxidant material is previously oxidized.
 3. A conductive antioxidant paint according to claim 1, wherein said antioxidant material is a carbide or nitride of an element selected from the group consisting of B, Si, Ge, Sb, Ti, Sn, Al and Zr, a boron element or a silicon element.
 4. A conductive antioxidant paint according to claim 1, wherein said inorganic colloid has an average particle size of not more than 100 nm.
 5. A conductive antioxidant paint according to claim 1, wherein said transition metal is at least one element selected from the group consisting of Cr, W, Co, Ti and Ni.
 6. A conductive antioxidant paint comprising a conductive material, an antioxidant material, a polymer emulsion and an inorganic colloid as a binder, and a transition metal, the content of alkali metal and/or alkali earth metal being not more than 20% by weight based on the weight of the antioxidant material.
 7. A conductive antioxidant paint according to claim 6, wherein said antioxidant material is previously oxidized.
 8. A conductive antioxidant paint according to claim 6, wherein said antioxidant material is a carbide or nitride of an element selected from the group consisting of B, Si, Ge, Sb, Ti, Sn, Al and Zr, a boron element or a silicon element.
 9. A conductive antioxidant paint according to claim 6, wherein said inorganic colloid has an average particle size of not more than 100 nm.
 10. A conductive antioxidant paint according to claim 6, wherein said transition metal is at least one element selected from the group consisting of Cr, W, Co, Ti and Ni.
 11. A conductive antioxidant paint comprising a conductive material, an antioxidant material and a binder; and having a total content of aluminum and silicon elements of not more than 1% by weight based on the weight of a solid content of the paint.
 12. A conductive antioxidant paint according to claim 11, wherein said antioxidant material is previously oxidized.
 13. A conductive antioxidant paint according to claim 11, wherein said antioxidant material is a carbide or nitride of an element selected from the group consisting of B, Si, Ge, Sb, Ti, Sn, Al and Zr, a boron element or a silicon element.
 14. A conductive antioxidant paint according to claim 11, further comprising an inorganic colloid having an average particle size of not more than 100 nm.
 15. A conductive antioxidant paint according to claim 11, further comprising at least one transition metal selected from the group consisting of Cr, W, Co, Ti and Ni.
 16. A graphite electrode coated with the conductive antioxidant paint as defined in claims 1, 6 or
 11. 